The complex nature of biological systems in general has made it impossible for researchers to determine directly the mode of metal-O2 bonding and factors which affect oxygen transport in the natural oxygen carriers. Therefore, the study of transition metal complexes as simple models continues to afford a testing ground for investigation of the factors and mechanisms associated with oxygen uptake and release at metal ion centers. We have recently shown that certain tetradentate thioiminatocobalt (II) complexes combine reversibly the with molecular oxygen in non-aqueous solutions, but the 1:1 adducts which form are less stable than those of the corresponding ketoiminato complexes. In order to determine if the presence of sulfur atoms in the ligand always hinders combination with molecular oxygen, we propose to synthesize and study the thermodynamics of oxygenation of several new cobalt(II) complexes which contain pentadentate thioiminato and mercaptobenzaldiminato ligands. The oxygen reactivity of selected macrocyclic cobalt(II) complexes with tetraaza ligands will also be studied in water and other protic solvents. EPR spectral measurements will be made to distinguish between 1:1 and 2:1 type adducts. Both variable temperature EPR and visible spectral methods will be used to obtain thermodynamic parameters for the oxygen adducts. Comparisons of the values obtained in this study with those reported for other oxygen adducts should enable us to evaluate the relative importance of pi-bonding vs. polarizability effects in the sulfur systems and the importance of ring charge and bond conjugation in the macrocyclic systems.